Hydrozoans or hydroids (Class Hydrozoa) are cnidarians, related to anemones and corals. There are a number of solitary hydroids but most are colonial, consisting of several types of specialised polyps or "hydranths". Although they come in a diverse range of forms, the hydranths of most colonial hydroids are connected via a branch-like "hydrocaulus". Hydroids are carnivores, catching prey with their stinging tentacles.

Athecate Hydroids (Order Anthoathecata)

Thecate Hydroids (Order Leptothecata)